[tsmon] Fix a test failing on go1.15.
[tsmon] Fix a test failing on go1.15.

The error was:
  http: RoundTripper implementation (...) returned a *Response with
  content length 8 but a nil Body

R=71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org

Change-Id: I669578f0b02476a26957ae2add1d73952e3dc6d3
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2351364
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Auto-Submit: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go